在Main.storyboard上拖入pickerview控件，设置一下大小和位置后，开始在
ViewController.swift里面写入代码

//遵循UIPickerViewDelegate，UIPickerViewDataSource协议，所以添加以下类
class ViewController: UIViewController, UIPickerViewDelegate, UIPickerViewDataSource {
}

//在预加载类里面写入如下，就是你打开应用的时候，程序首要加载的视图代码
override func viewDidLoad() {
super.viewDidLoad()
pickerView.delegate = self
pickerView.dataSource = self
}

//PickerView控件的值
let colors = ["Red","Yellow","Green","Blue"]

//colors 数组就是PickerView的数据源（data source）UIPickerViewDataSource 协议需要特定的方法来定义 picker 的组件和行数


func numberOfComponents(in pickerView: UIPickerView) -> Int {
return 1
}
func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
return colors.count
}

//定义了 picker 的行数等于数组的元素数量。接下来，将数组对应的元素内容赋值给对应的行

func pickerView(_ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? {
return colors[row]
}
//通过didSelectRow 给全局变量赋值，赋之后可以直接给后续逻辑调用
func pickerView(_ select_url: UIPickerView, didSelectRow row: Int, inComponent component: Int)
    {
        url_string = self.url_addr_select[row]
}